Among cellulose esters, cellulose acetate is known to be capable of application to optical films having a wide range of retardation values by varying its acetyl substitution degree. In general speaking, a triacetyl cellulose film having a high acetyl substitution degree has been preferably used as a protective film of a polarizing plate because of its low retardation. However, when a cellulose acetate film is used as an optical compensation film for various liquid crystal modes such as a VA mode or a TN mode, it has been necessary to add a retardation increasing agent since the retardation had been insufficient (for example, refer to Patent Document 1).
Since diacetyl cellulose having a lower acetyl substitution degree has a higher ability to provide a larger retardation value, it can be expected to be used as an optical compensation film without adding a retardation increasing agent. However, a cellulose resin having a lower acetyl substitution degree exhibits a higher moisture permeability due to its too high hydrophilic nature. Accordingly, it has been difficult to use diacetyl cellulose as a polarizing plate protective film as it is. Also, there has been a problem that a part of the film is dissolved into a saponofication liquid in the alkaline saponification step in the polarizing plate manufacturing process.
A technique to add a saccharide ester (sugar ester) compound into a cellulose acylate film for the purpose of reducing the moisture permeability has been, disclosed (for example, refer to Patent Document 2). However, in the above technique, it is expected be applied for a cellulose acylate film having an acetyl substitution degree of 2.6 or more, and, when the technique is applied for a diacetyl cellulose having further lower acetyl substitution degree, there has been a problem that haze of the film is increased. Accordingly, improvement has been desired.